1. Field of the Invention
The present invention relates to a system for controlling an industrial robot. More particularly, it relates to a system for controlling the rotation of a robot wrist having a servomechanism.
2. Description of the Prior Art
Generally, in industrial robots, the wrist and hand are attached to the end of an arm by means through of a joint, that is, a bevel gear. Workpieces or parts are grasped by the hand. The wrist is rotatable with respect to the arm, and such rotation is effected by the use of a servomechanism. For example, the wrist is driven to rotate by a direct-current motor. The output shaft of the direct-current motor is provided with a pulse coder. The output pulses of the pulse coder are fed back to a motor-drive control circuit.
A prior art system for controlling an industrial robot having such a servomechanism has a stroke-limit position control means to define the rotation of the wrist with, in a predetermined range. That is, a pulse coder is provided on the output shaft of the wrist-driving direct-current motor. The pulse coder produces a pulse count which is controlled within a range between a predetermined minimum value and a predetermined maximum value.
In the above-mentioned prior art, however, when the robot is performing operations for rotating parts such as screws, the control system is not suitable.